Stone fruits such as almond, apricot, cherry, nectarine, peach, and plum are important perennial fruit crops in the US and around the world. There is an increasing emphasis on producing larger fruit of high quality, as opposed to volume of fruit (tonnage). Growers are now challenged to produce crops of uniformly large fruit with adequate color and optimal flavor as consumers have grown to expect high quality fruit on a year round basis.
Reduction of the crop load on a tree (thinning) is often used to produce high quality tree fruit. During flowering and fruit set, growers commonly physically or chemically remove flowers (flower thinning) or young fruit (fruitlet thinning) to maximize the size and quality of the remaining fruit (Dennis, 2000, Plant Growth Reg. 31: 1-16). In general, the earlier the crop load is ‘thinned’ the better the quality of fruit at harvest. Removal of flowers or fruitlets on each tree by hand (hand thinning) often provides consistent results but can be prohibitively expensive. The use of chemicals for cost-effective flower or fruitlet thinning is preferable. The chemical insecticide carbaryl is often used for thinning apple fruitlets (Petracek et al., 2003, HortScience. 38: 937-942). However, carbaryl faces regulatory challenges and is no longer available to growers in some regions. The cytokinin 6-benzyladenine (6BA) is an important thinning chemical and is particularly effective for increasing fruit size. However, 6BA-induced thinning is sensitive to physiological and weather conditions (Yuan and Greene, 2000, J. Amer. Soc. Hort. Sci. 125: 169-176). For stone fruit such as peaches, there are currently no chemicals that safely and consistently induce post-bloom thinning (Costa and Vizzotto, 2000, Plant Growth Reg. 31: 113-119; Byers et al., 2003. In: Janick ed. Horticultural Reviews, John Wiley and Sons, Inc., 351-391). As Byers stated in 1978 (J. Amer. Soc. Hort. Sci. 103:232-236) “The search for an effective chemical peach thinning agent has not resulted in a commercially acceptable method of fruit removal. Numerous materials have been tried and most have been discarded due to inconsistent results, leaf abscission, fruit deformation, or unacceptable timing in relation to bloom and the frost period.” After more than 30 years since this publication, there is still a need for new chemicals that safely and consistently reduce crop load in these and other tree fruit crops.
Ethylene and ethylene-releasing compounds have been tested for apple, pear, and stone fruit thinning. Ethephon is a commercially available ethylene-releasing agent that has been extensively evaluated as a stone fruit thinner. Unfortunately, ethephon produces unreliable thinning results in part because ethylene release from ethephon degradation is inconsistent. Among factors, temperature greatly affects ethylene release rates (Wittenbach and Bukovac 1973, J. Amer. Soc. Hort. Sci. 98: 348-351; Olien and Bukovac, 1978, J. Amer. Soc. Hort. Sci. 103: 199-202; Olien and Bukovac 1982 HortScience 17: 795-796). For example, ethylene evolution from ethephon-treated sweet cherry shoots increased 4- to 6-fold when temperature increased from 20 to 30 C (Olien and Bukovac, 1978, J. Amer. Soc. Hort. Sci. 103: 199-202). Since temperatures change rapidly during the thinning season, predicting the appropriate ethephon dose is difficult and often results in under- or over-thinning (Olien, W. C. and M. J. Bukovac, 1978, J. Amer. Soc. Hort. Sci. 103: 199-202; Olien, W. C. and M. J. Bukovac, 1982, J. Amer. Soc. Hort. Sci. 107: 1085-1089; Cawthon et al. 1984, Texas PR 4206. Texas Agric. Expr. Stn., Overton, Tex.).
In addition to an unpredictable thinning response, ethephon may cause significant defoliation, even when thinning is only moderate (Gambrell et al. 1983. J. Amer. Soc. Hort. Sci. 108: 605-608). Ethephon also has been shown to reduce final fruit size (Weinbaum et al., 1977. J. Amer. Soc. Hort. Sci. 102: 781-785) which negates one of the major desired quality outcomes of thinning. Moreover, ethephon application has been implicated in causing a physiological disorder called gummosis (Olien and Bukovac, 1983, Acta Hort. 137: 55-64). Gummosis is a generalized disorder of trees in which polysaccharide gum is overproduced, exuded, and deposited on the bark. Gummosis affects water relations, promotes disease, is attractive to wood-boring insects, causes shoot death, and leads to early tree decline.
Thus, although ethephon has provided some positive results, it is not currently registered for this use because of inconsistent thinning due to environmental and physiological factors, extensive leaf drop, and stimulation of gummosis (Costa and Vizzotto, 2000. Plant Growth Reg. 31: 113-119; Byers et al., 2003, Horticultural Reviews, J. Janick, ed. John Wiley and Sons, Inc. p. 351-391; Olien and Bukovac, 1983, Acta Hort. 137: 55-64). Consequently the external application of ethylene to trees (e.g. through the use of products like ethephon which are sprayed onto trees) is not recommended as a thinning agent for stone fruit.
It is an object of this invention to reduce the crop load of stone fruits, such as peaches, using ACC during the growing season.
It is an object of this invention to reduce the crop load of pome fruits, such as apples, using ACC alone or in combination with other thinners, applying it during the growing season.
It is a further object of the present invention to use ACC to increase the quality of stone fruit.
It is a further object of the present invention to use ACC and ethephon combinations to increase the quality of stone fruit.
It is an object of this invention to show the unexpected finding that induction of the plants ethylene production by ACC is inhibited at 35° C. or higher temperatures.
It is an object of this invention to demonstrate that ethephon and ACC have differential ethylene release profiles and differential physiological effects in response to temperature.